Conventionally, in the field of medical diagnosis, an imaging apparatus using a film, an image-intensifier type imaging apparatus, etc. have been used as means for visualizing an X-ray image. In resent years, development of a flat-panel type X-ray imaging apparatus as a new imaging apparatus substituting the above apparatuses is active, and clinical experiments have been partly started.
This flat-panel type X-ray imaging apparatus uses, as a key device, a flat-panel type X-ray detector which is produced by combining a large-area thin-film transistor array technique used in an active matrix type liquid crystal display device and an X-ray conversion film technique for converting X-rays into electrical signals, and has various advantages over conventional X-ray imaging apparatuses. More specifically, the flat-panel type X-ray imaging apparatus achieves an improvement of the image quality and diagnosis support by digital image processing as well as instantaneous conversion of a result of imaging into image signals and display of the result on a display or output of the result to a printer, and easily stores and transfers the result of imaging as digital image information, compared with a conventional film-type imaging apparatus. Moreover, compared with the conventional image-intensifier type imaging apparatus, the flat-panel type X-ray imaging apparatus achieves a significant reduction in its thickness, and provides a large-area, high-resolution X-ray image.
The following description will explain the structure and operational principle of the flat-panel type X-ray imaging apparatus. The specific structure and properties of the flat-panel type X-ray detector are described in detail in documents, for example:
Denny L. Lee, et al., "A new digital detector for projection radiography", SPIE, Vol.2432, pp237-249, 1995; and
Wei Zhao, et al., "A flat panel detector for digital radiology using active matrix readout of amorphous selenium", SPIE, vol. 2708, pp523-531, 1996.
FIG. 18 shows an example of a conventional X-ray imaging apparatus using a flat-panel type X-ray detector. The X-ray imaging apparatus includes an X-ray generator 51, a flat-panel type X-ray detector 52, a control device 53, an operator console 54, an image processing device 55, a display device 56, and a printer device 57.
X-rays emitted from the X-ray generator 51 pass through a specimen 58, and are incident on the flat-panel type X-ray detector 52. The incident X-rays are converted into a two-dimensional charge distribution according to the quantity of the incident X-rays, further converted into digital image signals and sequentially output. The digital image signals are subjected to image processing, such as a gray-scale correction, in the image processing device 55, and sent as display signals to the display device 56 where the signals are visualized (displayed). Moreover, the digital image signals are sent to the printer device 57 and output as a print, if necessary. Although not shown in FIG. 18, it is possible to store digital image data in an image storage device or transmit the digital image data to a remote place.
The operator console 54 is provided with a switch for allowing an operator to instruct an irradiation start timing of X-rays, and X-ray imaging is started upon the operation of the switch. Besides, the control device 53 controls the entire sequence and timing.
By the way, the conversion method of the flat-panel type X-ray detector 52 is roughly classified into two types: a direct conversion method of directly converting X-rays into charge by a conversion layer; and an indirect conversion method of converting X-rays into light temporarily using a scintillator and then converting the light signals into electrical signals by a photodiode. The indirect conversion method is disclosed in, for example, Japanese laid-open patent application No. (Tokukaisho) 62-2933 (published Jan. 8, 1987). Here, for the sake of convenience of explanation, the following description will illustrate the direct conversion method.
FIG. 19 shows a schematic structure of essential sections of the flat-panel type X-ray detector 52. A number of pixels 61 are arranged in a matrix form, and each pixel 61 is connected to a scanning line SLj (j=1 to m: m is an integer of not less than 2) and a signal line DLi (i=1 to n: n is an integer of not less than 2) through a TFT 72 (see FIG. 21) as a later-described switching element. Each scanning signal SLj is connected to a scanning line drive circuit 62, while each signal line DLi is connected to a signal readout circuit 63. The scanning line drive circuit 62 and signal readout circuit 63 are controlled by a timing control circuit 64.
As the scanning line drive circuit 62, a gate driver IC (Integrated Circuit) used in a general liquid crystal display device can be used. Besides, as shown in FIG. 20, for example, the signal readout circuit 63 includes: pre-amplifiers 65 which are provided in association with the signal lines DLi, respectively, and perform voltage conversion and amplification of input signals; a multiplexer 66 for switching the outputs from the pre-amplifiers 65 consecutively to an A/D converter 67 in a later stage; and the A/D converter 67 for converting analog image signals from the multiplexer 66 into digital image signals.
FIG. 21 depicts an example of a cross sectional structure of the pixel 61 of the flat-panel type X-ray detector 52 (see FIG. 18). FIG. 22 shows an equivalent circuit of the pixel 61. As illustrated in FIG. 21, the pixel 61 includes a TFT (thin film transistor) 72 formed on a glass substrate 71, a storage capacitor (Cs) 73, etc.
The TFT 72 includes a gate electrode 74 connected to the scanning line SLj, a gate insulating film 75 formed to cover the gate electrode 74, a source electrode 76 and a drain electrode 77 formed on the gate insulating film 75. The source electrode 76 is connected to the signal line DLi, while the drain electrode 77 is connected to a pixel electrode 78.
The storage capacitor 73 is configured such that the pixel electrode 78 and a lower common electrode 80 connected to the negative terminal of a bias power supply 79 face each other with an insulating film 81 therebetween. Additionally, a charge preventing layer 82 is formed to cover the source electrode 76, drain electrode 77 and pixel electrode 78.
As a TFT matrix formed by the TFT 72 and storage capacitor 73, it is possible to use a TFT substrate which is produced in the process of manufacturing an active matrix type liquid crystal display device. For instance, a TFT substrate for use in an amorphous silicone (a-Si) TFT liquid crystal display device has scanning lines, signal lines, storage capacitors, etc., and can be used as the TFT matrix of a flat-panel type X-ray detector 52 by making slight changes.
Further, in each pixel 61, a photoconductive film 83, a dielectric layer 84, and an upper common electrode 85 connected to the positive terminal of the bias power supply 79 are formed successively to cover the TFT 72 and storage capacitor 73, and a pixel capacitor Cp (see FIG. 22) is produced. As a material for the photoconductive film 83, a semiconductor material which absorbs X-rays and converts the X-rays into charge with high efficiency is used. For example, in the above documents, an amorphous selenium (a-Se) film formed in a thickness of 300 to 600 .mu.m by vacuum evaporation is used.
Next, the following description will explain the operation of the flat-panel type X-ray detector 52 of the above-mentioned structure.
X-rays incident on the photoconductive film 83 are absorbed within the photoconductive film 83, and converted into charge according to the quantity of the X-rays. Since the photoconductive film 83 and storage capacitor 73 structurally form a capacitor which is electrically connected in series, the generated charge (electrons and holes) moves to electrodes of different polarities, respectively, upon application of a bias voltage across the upper common electrode 85 and the lower common electrode 80 by the bias power supply 79, thereby storing predetermined charge in the storage capacitor 73. Hence, the photoconductive film 83 and storage capacitor 73 form an X-ray detecting element which converts X-rays passed through the specimen 58 (see FIG. 18) into charge and stores the charge.
The charge stored in the storage capacitor 73 can be removed from the storage capacitor 73 through the signal line DLi by application of a voltage sufficient for turning on the TFT 72 to the scanning line SLj. Therefore, as illustrated in FIG. 19, by performing line sequential scanning of the scanning lines SLj with the scanning line drive circuit 62, signals over the entire pixels 61 can be obtained. The signals extracted from the pixels 61 are subjected to voltage conversion, amplification, and A/D conversion in the signal readout circuit 63 connected to each column of the signal lines DLi, and the information of the X-ray image is detected as digital image signals.
By the way, the readout operation of the image signals is roughly classified into two categories, according to the objects of flat-panel type X-ray imaging apparatus. One object is to replace an imaging apparatus using an X-ray film with a flat-panel type X-ray imaging apparatus. The other object is to replace an image-intensifier type X-ray imaging apparatus with a flat-panel type x-ray imaging apparatus. Here, the operation associated with the former object is called the "radiography mode", while the operation associated with the latter object is called the "fluoroscopy mode". In other words, a still image of the specimen is obtained in the radiography mode, while a moving image of the specimen is obtained in the fluoroscopy mode.
In the radiography mode, a resolution as high as or higher than the current X-ray film, low noise, and a wide dynamic range are required. In this mode, a relatively large quantity of X-rays of several mR per frame may be irradiated, and a readout time of up to several seconds is also allowed. Thus, this mode can be relatively easily achieved.
On the other hand, in the fluoroscopy mode, a high-speed imaging of around 30 frames per second is required, and it is, for example, necessary to image a movement of the heart of an infant and a movement of a taken contrast medium in the esophagus in real time. In this mode, considering the exposure of X-rays, the quantity of X-rays which can be irradiated on the specimen is as small as several tens .mu.R or less per frame. Therefore, in the fluoroscopy mode, it is necessary to perform imaging with extremely high sensitivity and low noise. It is thus difficult to achieve the fluoroscopy mode compared with the radiography mode.
As measures to easily achieve the fluoroscopy mode, development of a photoconductive film with a high X-ray conversion efficiency and an attempt to improve the X-ray conversion efficiency by increasing the thickness of the photoconductive film have been carried out from the aspect of the device.
On the other hand, from the aspect of the drive circuit, a proposal to perform high-speed scanning by driving a plurality of scanning lines simultaneously to increase the quantity of signals instead of sacrificing the resolution has been made. When simultaneous driving of a plurality of scanning lines is performed, since the signals from a plurality of pixels are extracted to the respective signal lines, the quantity of signals is increased compared with driving of a single scanning line. For example, if the amount of charge stored in the storage capacitor of each of the pixels is uniform, when two scanning lines are driven simultaneously, the quantity of signals extracted to the signal lines is two times that extracted by driving a single scanning line. Similarly, when three scanning lines are driven simultaneously, three times the quantity of signals extracted by driving a single scanning line is extracted to the signal lines.
By the way, in practice, there is a problem in driving a plurality of scanning lines simultaneously. Specifically, since a parasitic capacitance exists between the scanning line and signal line, a significant amount of charge moves to the signal line through the parasitic capacitance upon a transition of the voltage of the scanning line, and this effect becomes stronger with an increase in the number of scanning lines which are scanned simultaneously.
The degree of the effect can be estimated as follows. As the parasitic capacitance between the scanning line and signal line, there are mainly the parasitic capacitance at a crossed lines section, parasitic capacitance between the gate and source and between the gate and drain. These parasitic capacitances usually amount to a total of around 0.02 to 0.06 pF. Besides, as the drive voltage of the scanning line, an amplitude of around 15 to 25 V is generally selected to sufficiently turn on/off the TFT. Therefore, a variation of the charge on the signal line side due to the transition of the voltage of a single scanning line is around 0.3 to 1.5 pC. This variation of charge is two-digit scale larger than a maximum signal charge (utmost several fC to tens of several fC) expected for each pixel in the fluoroscopy mode.
In the case of a TFT array having 2000 or more pixels per side of an around 40 cm square screen, when a general process is used, the resistance of the overall length of the scanning lines is around 10 k.OMEGA. and the capacitance is around 100 pF. Therefore, the drive waveform on the scanning line varies according to the distance from the scanning line drive circuit, and a transient response of the variation of charge on the signal line side during the transition of voltage of the scanning line also varies according to the distance between the scanning line drive circuit and each signal line. Hence, when the variation of charge on the signal line side due to the transition of the scanning line drive voltage is increased, since the non-uniformity in a scanning line direction is increased, it is necessary to take a countermeasure against the non-uniformity in a scanning line direction.
As described above, simultaneous driving of a plurality of scanning lines increases the variation of the voltage associated with the driving. As a countermeasure, it is necessary to perform a special offset adjustment and delay the time of signal sampling in the signal readout circuit. As a result, the circuit becomes complicated, and noise due to fluctuations in the scanning line voltage is increased.